Hi,
I bought my rats around 3 months ago after a lot of research into their care and training etc. I went to a local pet shop, but they only had boys (which I was happy with, partner didn’t like their balls) so off we went to pets at home to get girl rats.
Alarm bells first started ringing when they couldn’t catch the bloody things cause they were so skittish. The male ones we had seen earlier came straight to the pet shop worker and were very calm.
We’ve been trying for ages at getting them out the cage but they just don’t want to be touched. They’ll take food out of our hands, but if we leave the door open they’ll poke their head out and hide again. If you go to stroke them they hide and one of them isn’t too fussed about taking a chunk of skin.
They’ve recently also stopped using their litter tray and peeing alive the wooden shelving and down the side of their cage, so they’re getting smelly I clean the wood as well as I can, but I can’t get hold of them to clean them.

Does anyone have any good advice? We’re gutted we can’t get them out. (We got them out once, it took us 2 hours to catch them. I’m surprised they didn’t die of a heart attack to be quite honest!)

I've never had a rat that difficult to catch to be honest, but I found wearing thick gloves (winter gloves or gardening gloves) was helpful for catching skittish gerbils, so you could try that. Gloves make your hands effectively bigger and harder to avoid. It would give you some protection from bites and help you be more confident too. The more confident you are, the more quickly and firmly you'll feel able to move, and the more likely you are to be able to just grab them. You don't need to be too careful when handling them; rats are pretty robust.

Take out all the enclosed spaces from their cage; replace them with open toys (flat hammocks, perches, ropes, etc.) if possible. Forcing them to be out in the open can apparently help them acclimatise faster, since they can't just hide away anytime something scares them. If they haven't got things to hide in, grabbing them out of the cage should be easier too. Rearrange the cage however you need to make access as good as possible for you.

Normally people advise putting on a dressing gown and shoving them in there and just keeping them on your person for 20 minutes or so, but with rats this skittish I'd make sure to do so in a small, enclosed space so that they can be recaptured more easily if they dash away from you. Ideally, make a small playpen with walls at least ~60cm high (preferably higher if they're really jumpy), and sit in it. Usually a rat will be cautious about leaving a familiar person for less familiar floor, but with super-antisocial rats, I would be a bit more wary of the escape risk.

If they'll take food out of your hands that's a very positive avenue to explore. Try smearing something wet and tasty on your fingers, like baby food. Something they can't carry away with them and have to stay there to lick off you.

If/when you manage to catch them reasonably reliably, try just picking them up and putting them straight back down again. That will hopefully teach them that being picked up is no big deal.

Oh and you probably just want to take out the wooden shelves. Large flat surfaces pretty much always turn into rat toilets; if you do want to keep them in the cage, you'll need to paint them with a wipe-clean, child-safe paint (Plastikote enamel is what I use).

If you ever really need to get the rats out of the cage, and you have a playpen or other small secured area, you could just dismantle the cage with them in it. Take out all the toys and accessories, take the cage top off the base, and if they're clinging to the top then just put it on the floor and wait for them to get tired and go onto the floor.